This application is intended to systematically evaluate the role(s) of cytoplasmic fibers in the expression of cellular injuries caused by hyperthermia. We have reason(s) to believe that cytoplasmic fibers are one of the major targets involved in heat-induced cell-death as well as being responsible for the differences in thermal sensitivity between normal and tumor cells. These reasons are (1) Cytoplasmic fibers are key components in maintaining the structural and functional integrity of the cell and is an important component involved in cell division; (2) One of the cytoplasmic fiber components, namely microtubule, is known to be temperature sensitive. Limited available data as well as our own experiments are consistent with this fact; (3) The environmental factors such as pH, hypoxia, and ionic-conditions which greatly change the cellular response to hyperthermia also markedly affect cytoplasmic fibers; (4) Agents which disrupt microtubules are cytotoxic; (5) Agents which stabilize the microtubule protect cells from hyperthermic cytotoxicity; (6) The cell cycle specific cytotoxic effects between hyperthermia and microtubular disrupting agents are very similar; (7) Furthermore, ample evidence is available which indicates that there are apparent differences in cytoplasmic fibers between normal and transformed cells which may contribute to the selective sensitivity of hyperthermia toward transformed cells. Thus, to test our hypothesis, we propose to correlate the cytotoxicity of hyperthermia and cytoplasmic fiber organization in various cells under different conditions. To further assist us in determining this relationship we also plan to study: (1) the interaction between heat and agents which can modify cytoplasmic fibers; (2) the interaction between heat and cellular environments; and (3) the heat effect on purified cytoplasmic fiber proteins. Our endeavour, we believe, will shed a new light into the mechanism by which heat can injure cells and may lead to the understanding of differential thermal sensitivity to transformed cells. This information then can be utilized in improving hyperthermia as an anti-tumor agent.